McClear is one of the major outcome of the CAMS project, which contributes to a detailed assessment of optical variables in the atmosphere (gaz, aerosols, water vapour), clouds and ground albedo. The model exploited in McClear estimates the radiation that should be received if the sky were clear anywhere, since 2004. This model has been created by the Centre O.I.E., the DLR, the Finnish Meteorological Institute), and the European Center for Medium-range Weather Forecasts ECMWF.

The method Heliosat-4 processes Meteosat images to create the CAMS radiation service. This method is mainly based on the principle of separability between the clear sky radiation (CAMS McClear), and the effects of clouds (cloud properties from APOLLO, DLR) and ground albedo (derived from MODIS). Please take a look to the description of Heliosat-4 in three slides below.

CAMS radiation service covers Europe, Africa, the Mediterranean Basin, the Atlantic Ocean and part of the Indian Ocean (-66° to +66° both in latitude and longitude). CAMS radiation service delivers spectrally-integrated 15 minute Global Horizontal Irradiance (GHI) and Direct Normal Irradiance (DNI) values.

ground albedo impacts only the diffuse part of the radiation, that is why MODIS-derived ground albedo is only exploited to compute Global component ("G" in the previous slides) of CAMS Radiation service.

McClear is exploited by HelioClim-3 version 5. As HelioClim-3 stores Global Horizontal Irradiation values from which all other radiation components are derived using a series of models, MODIS ground albedo consequently impacts all components.

More information about APOLLO cloud properties (DLR)

Principle

APOLLO uses the multispectral information provided by the SEVIRI instrument of Meteosat Second Generation to derive cloud properties. From this information, DLR created a "mapping" with 4 cases of clouds of reference, associated to an average height and an average geometric thickness. A more precise description of this cloud classification and how it is exploited in Heliosat-4 is available page 88, section 5, of Z. Qu's PhD. Please find below the corresponding extract (in French, Google Trad is my friend :)):

APOLLO and the reliability code

The reliability variable is computed with respect to the amount of 1 minute resolution time slots of the current time slot (15 min or 1 hour) without cloud information. In the morning and evening, for very low sun elevation angles, the APOLLO cloud computation gives no results, and this explains the value lower than 1 for the first/last 15 min (or 1 hour) slot in the morning/evening. During the day, it happens from time to time that the cloud information cannot be computed for a given satellite images set and this also creates a lower reliability value (this can be caused by a problem in the satellite images reception or in the APOLLO computing process - CAMS radiation is still a bit "experimental"). Thus, in summary, the reliability value gives an idea of the amount of interpolated data for the time slot of interest.

Content: Presentation of the monthly maps of MODIS parameters (fiso, fvol and fgeo) used to generate albedo for each instant in the year, derived from holy maps of collected MODIS maps (holes due to clouds).